Processes for the disproportionation of toluene to produce benzene are well-known in industrial applications. Recently, however, the economic desirability of producing benzene has come under question. The effects of recent legislation, including the Clean Air Act, limit benzene levels in gasoline pools, which, in turn, may reduce the economic incentive to produce benzene. Consequently, these effects may cause an excessive amount of benzene in the market which may result in a decrease in the price of benzene.
In preparation for the regulations concerning a reformulated gasoline pool, including dropping the aromatic levels and removing benzene and xylene, a catalytic process using a mordenite catalyst has been developed to convert benzene and C.sub.8 aromatics, such as xylene and ethylbenzene, to toluene. The toluene can be added to the gasoline pool for octane boost. This process involves transfer of the alkyl group between xylene(s) and benzene, producing toluene as a major product and other alkylated hydrocarbons (C9+).
The conversion of a mixed alkylaromatic feed comprising benzene, xylene(s) and, optionally, ethylbenzene to form toluene may be practiced in accordance with the following reaction: ##STR1##
As carried out in the instant invention, reaction (1) employs a metal-loaded mordenite zeolite catalyst. Ethylbenzene may be employed as an optional feedstock component.
Mordenite is one of a number of molecular sieve catalysts useful in the conversion of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite exhibiting a network of silicon and aluminum atoms interlinked by oxygen atoms within the crystalline structure. For a general description of mordenite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981, under the heading "Molecular Sieves", Vol 15, pages 638-643.
Mordenite as found in nature, or as synthesized to replicate the naturally occurring zeolite, typically exhibits a relatively low silica to alumina mole ratio of about 10 or less. Also known are "aluminum-deficient" mordenite catalysts exhibiting silica to alumina ratios between 10 and 100, which may be prepared by direct synthesis as disclosed in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies, et. al. Both the typical and the aluminum deficient mordenites are known to be useful in toluene disproportionation reactions.
It is also a common practice to promote (or load) an aluminum deficient mordenite catalyst with a catalytically active metallic component. For example, U.S. Pat. No. 3,476,821 to Brandenburg et al. discloses disproportionation reactions employing mordenite catalysts having silica/alumina ratios within the range of 10-100 and preferably within the range of about 20-60. The mordenites are modified by the inclusion of a sulfided metal selected from the Group VIII metals. The metal may be included in the mordenite by well known ion exchange or impregnation techniques. The metal promoters substantially increase activity and catalyst life, as indicated by runs extending over several hours or days.
Other patents, such as U.S. Pat. No. 4,956,511 to Butler, U.S. Pat. No. 4,761,514 to Menard, U.S. Pat. No. 3,562,345 to Mitsthe and U.S. Pat. No. 3,677,973 to Mitsthe et al., disclose the use of molecular sieves such as mordenite catalysts in the disproportionation of toluene, and each of the entire disclosures of the above-referenced patents are incorporated herein by reference.
While metal-promoted mordenite catalysts may typically be used in toluene disproportionation processes, it has become desirable to employ these catalysts in a reverse toluene disproportionation process, i.e., toluene synthesis. Given the facts that (1) gasoline specifications will limit benzene levels and (2) toluene could be effectively used as a substitute for benzene, a benzene surplus may result causing the economic value of toluene to rise. It has therefore become desirable to develop a process for the conversion of surplus benzene and xylenes to toluene.